1. Field of the Invention
The present invention relates to a bearing device for use in a canned motor, and more particularly to a bearing device comprising a cartridge-type bearing for use in a canned motor, and a canned motor equipped with such a bearing device.
2. Description of the Related Art
A canned-motor pump has a pump casing for housing a canned motor therein. In general, a canned-motor pump circulates some of the fluid being pumped to lubricate the bearings of the canned motor and also cool the canned motor. The bearings of the canned-motor generally include two radial bearings of plain metal for supporting a motor rotor at its opposite ends and two thrust bearings of plain metal for bearing thrust loads applied to both side ends in opposite axial directions. It is important that these bearings be highly accurate both individually and relatively to each other, i.e., with respect to mutual concentricity, perpendicularity to the motor axis, axial end play, and other factors. To meet such accuracy requirements, it is necessary for the bearings to be structured and shaped for easily achieving a desired level of accuracy, so that the bearings can be machined and assembled with accuracy.
One conventional bearing device assembled in a canned motor will be described below with reference to FIGS. 12, 13(a) and 13(b) of the accompanying drawings.
As shown in FIG. 12, a canned motor has a rotor 60 rotatably supported by radial bearings 62, 63 on both ends thereof. The rotor 60 has a shaft 61 on which thrust disks 64, 65 are fixedly mounted at both ends, respectively. Thrust bearings 66, 67 are fixed to respective axially outer surfaces of the respective thrust discs 64, 65. The canned motor also has a motor frame 68 to which a thrust bearing 69 is fixed near the output side end, the thrust bearing 66 being rotatable with respect to, and disposed in axially confronting relationship to, the thrust bearing 69. The radial bearing 63 is supported in a bearing housing 70 that is fastened to the motor frame 68 near the other end. The thrust bearing 67 is rotatable with respect to, and disposed in axially confronting relationship to, the radial bearing 63. The motor frame 68 and other parts fixedly coupled thereto serve as a motor stator.
The canned motor is incorporated in a pump for delivering a liquid. If such a liquid contains a slurry, the clearance between the sliding surfaces of each of the radial bearings 62, 63 should be small to prevent the slurry from entering the radial bearings 62, 63. The smaller the clearance between the sliding surfaces of each of the radial bearings 62, 63, the higher the pressure between those sliding surfaces, making it more difficult for the slurry to find its way into the radial bearings 62, 63. To achieve a desired accuracy of the clearance between the radial bearing surfaces, it is necessary that the concentricity of the radial bearings 62, 63 be highly accurate. However, since the desired concentricity requires precision machining of both side ends of the motor frame 68 the cost of the canned motor is relatively high.
If the axial end play of the thrust bearings 66, 67, 69 is too large, the slurry tends to enter the thrust bearings 66, 67, 69 while the pump is not in operation, and the thrust bearings 66, 67, 69 are liable to be damaged due to shocks that may occur during shipment. Consequently, the thrust bearings 66, 67, 69 are required to have an optimum axial end play.
To obtain such an optimum axial end play in the thrust bearings 66, 67, 69, the canned motor is required to accurately define a dimension A (see FIG. 13(a)), i.e., the distance between the axially outer surfaces of the rotatable thrust bearings 66, 67, and a dimension G (see FIG. 13(b)), i.e., the distance between the axially inner surfaces of the stationary thrust bearing 69 and the radial bearing 63. To accurately define the dimension A, it is then necessary to accurately define the dimensions of a plurality of parts that are involved in determining the dimension A as shown in FIG. 13(a). Similarly, to accurately define the dimension G, it is then necessary to accurately define the dimensions of a plurality of parts that are involved in determining the dimension G as shown in FIG. 13(b).
The shaft 61, which is a relatively large part of the rotor, cannot easily be machined because its large dimension has to be defined with the same accuracy as the other parts.
Likewise, the stator, which is also a relatively large part of the motor, cannot easily be machined because its large dimension has to be defined with the same accuracy as the other parts and also because the stator itself requires a high machining finish.
In addition, since the thrust bearings 66, 67, 69 are positioned on the opposite ends of the shaft 61, the actual axial end play in those thrust bearings 66, 67, 69 can be determined only after the rotor is actually assembled in the stator. In the event that the actual axial end play is found to be inappropriate, the rotor has to be disassembled from the stator, the thrust bearings 66, 67, 69 and other parts have to be machined or adjusted, and the rotor has to be assembled in the stator again. Such disassembling, adjusting, and reassembling processes have to be repeated until the proper axial end play is achieved.